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BIG PHYSICS, BIG QUESTIONS –

‘Tenth planet’ is bigger than Pluto

By Hazel Muir

Xena is amazingly distant, roughly three times as far from the sun as Pluto (Artist’s impression: NASA/JPL-Caltech)

The chilly world dubbed Xena on the outskirts of the solar system has as at least as much claim to be a planet as Pluto, according to a new study confirming that the “tenth planet” is by far the larger of the two.

Astronomers first spotted Xena, known more formally as UB 313, in 2003, but the discovery was not made public until July 2005. By then they realised it was amazingly distant – at times, about three times as far from the Sun as Pluto is.

From its brightness and distance, astronomers estimated that it must be slightly larger than Pluto – assuming that, like Pluto, it reflects just over half the sunlight that falls onto it.

To calculate Xena’s size more directly, Frank Bertoldi of the University of Bonn in Germany and his colleagues measured its light at a wavelength of 1.2 millimetres using a telescope array in the Spanish Sierra Nevada. Xena emits this radiation in response to being warmed by the Sun, regardless of the shininess of its surface.

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What’s in a name?

“These numbers are very believable,” says Mike Brown of the California Institute of Technology in Pasadena, US, whose team discovered the distant body. “We knew from day one that it was definitely bigger than Pluto.”

However, Brown points out that the margin of error in the new result is fairly large, at around 400 kilometres. He is currently analysing new images from the Hubble Space Telescope that may reveal Xena’s size to the nearest 50 kilometres, and he hopes to announce the result in about a month.

On whether Xena should be classified as a planet, Bertoldi says&colon; “Pluto is culturally known as a planet, so why not call this a planet too? Scientists know exactly what it is, so we really don’t have to have any debate.”